1. Field
This invention relates to improvements in semiconductor devices, and in particular, to semiconductor diodes designed for operation at millimeter and submillimeter wave frequencies.
2. Prior Art
Characteristics useful for the development of a high quality diodes intended for millimeter and submillimeter wave frequency operation include the ability to accurately predict skin effect contribution near the operation frequency from theoretical calculation based on device geometry and the ability to verify these calculations by low frequency measurements of the devices. Desirable device characteristics required for high performance mixer applications at these frequencies include a low value of junction capacitance at zero volt bias, a minimum of variation in junction capacitance as a function of bias, a low value of series resistance, a low value of parasitic capacitance, and nearly unity ideality factor.
The ideality factor is a measure of the perfection of the junction and is derived from the diode current equation. Diode current is given by: ##EQU1## where I is the diode current, I.sub.s is the saturation current and N is the ideality factor. A perfect diode will have an ideality factor of unity.
In prior art devices, such as planar point contact devices and planar beam lead devices, the combination of all the above characteristics are not present in any one device.
In addition, epitaxial N-layers on N+substrates are currently being adopted universally as the basic substrate material for such devices. The disadvantages of this type of substrate include restricting fabrication to a single device on a simple substrate, the necessity of measuring ohmic series resistance at or near the operating frequency, a difficult and often impossible measurement at 100 GHz or higher frequency, poor reliability due to inherent, premature avalanche breakdown, and poor collection of current resulting in higher series resistance.
Prior art structures also suffer from the use of very thin silicon dioxide layers which are used to provide passivation, support of the beam leads and a surface on which to first define the junction area. Unfortunately, a thin silicon dioxide layer does not provide sufficient passivation protection for a highly reliable device and the thin passivation layer also results in high parasitic capacitance between the beam leads and the N+layer.
For millimeter and submillimeter wave frequencies applications, the junction diameter can be in the low millimeter range. To produce such a small junction using planar technology, a thin passivation layer is necessary because the junction is defined photolithographically on the passivation layer. The passivation layer is then etched through to the active layer to provide for the deposition of the metal contacting layer which forms the junction. If the passivation layer were made thick, the junction would be ill defined because the etching would not proceed uniformly through a thick passivation layer.